Extractive distillation of alcohols with sulfolane



Oct. 9, 1951 c. s. CARLSON ETAL 570 205 EXTRACTIVE DISTILLATION 0F ALCOHOLS WITH SULFOLANE.

Filed March 12, 1948 QEbomarL C ZS. I n. P551 v. 535 3dr.

b3 MAM-attain 2.3

Patented Oct. 9, 1951 CTIVE ms'rrnm'rron or ALcono wrrn sumomma Carl S. Carlson, Elizabeth, and Paul V. Smith, Jr., Westfleld, N. J., asslgnors to Standard Oil Development Company, a corporation of Delaware Application March 1:, 1m, Serial No. 14,550

9 Claims. (or. 202-395) This invention relates to a practical method of separating close-boiling oxygenated organic compoundsand is concerned with the controlled use of sulfolane as a refluxing medium in a continuous fractional distillation of the close-boiling oxygenated compounds.

In copending application, Serial No. 768,440, flied August 13, 1947, it is proposed to fractionate close-boiling oxygenated compounds by fractionally distilling the oxygenated compounds in the presence of a large excess of a hydrocarbon oll which is liquid under the conditions obtaining in the fractionation zone. In such a system the volatilities of the compounds are altered to such an extent that separations are possible which are diillcult to obtain by ordinary fractionation.

There are, however, a number of disadvantages in employing straight hydrocarbons as a reflux medium in separating such oxygenated compounds. In the first place,solubllity relationships are such that onlysmall amounts of water can be present without separation of two liquid phases. Such liquid phase separation is generally quite undesirable in such systems since it results in loss of selectivity. Furthermore, in order to prevent entrainment of the hydrocarbon overhead with the alcohol products, both in the fractionation stage and in the subsequent stage of stripping the alcohol from the hydrocarbon, it is necessary to use a hydrocarbon of relatively high initial boiling point. This high initial boiling point of the hydrocarbon results diene with sulfur dioxide and then hydrogenating the resulting product.

The process of this invention is best applied to distillation cuts or mixtures, the components of which distill within a narrow range; however, it may be applied to wide-boiling mixtures as well. The invention is particularly directed to the separation of alcohols of different types and molecular weights from one another and to the separation of alcohols as a class or one particular alcohol from other oxygenated com pounds such as ketones, acetals, esters, aldehydes. etc. Typical separations which can be made are ethyl from isopropyl alcohol, a mixture of ethyl and isopropyl alcohols from methyl ethyl ketone, ethyl from a mixture of isopropyl alcohol and methyl ethyl ketone, n-propyl alcohol from sec-butyl alcohol, n-propyl alcohol from i-butyl alcohol, n-propyl alcohol from a mixture of sec-butyl and i-butyl alcohols, n-propyl alco--' hol from methyl butyl ketone, n-propyl alcohol from valeraldehyde, n-propyl alcohol from a mixin high tower temperatures and in the necessity Sulfolane'may be prepared by condensing butature of C4 and higher molecular weight alcohols, n-propyl alcohol from a mixture of C5 and higher molecular weight carbonyl compounds, n-butyl alcohol from isomeric C5 and higher molecular weight alcohols, etc., a mixture of ethyl and isopropyl alcohols from other closely boiling oxygenated compounds, ethyl alcohol from a mixture of isopropyl alcohol with other oxygenated compounds and a mixture of alcohols from the oxygenated compounds.

The crude oxygenated mixture may contain amounts of water greater than, less than, or equal to the amounts corresponding to azeotropic compositions, but in anycase it must be miscible with the solvent in all portions of the fractiona tion zone.

Some of the above-described mixtures are obtained by an olefin hydration reaction, e. g., when a mixture of ethylene and propylene is absorbed in sulfuric acid, diluted, hydrolyzed and a resulting aqueous alcohol mixture is stripped out. Another important source of such mixtures is the Fischer synthesis hydrogenation of carbon monoxide, especially when the aqueous layer product formed contains not only lower primary and secondary alcohols but also various ketones. aldehydes, ethers, acetals, esters. carboxylic acids and certain tertiary alcohols. Still another source is in the products of, hydrocarbon oxidaition where both oil and water layers are obtained, both containing oxygenated organic compounds.

66 Narrow-boiling range mixtures which may be ob 3 tained by the ordinary distillation process from aqueous solution are as follows:

TABLE I Narrow-boiling range alcohol mixtures Normal Group Components B Pt.,

7a 82 7a 82. 1 79. 7s. .82. ea 1a;

82 10a 91. 9o. 91. no. 107. 91. 99. 121. 111. 119.

The narrow-boilingrange mixture may be a binary or tertiary mixture as in the groups shown, biit generally the crude mixtures contain additional oxygenated organic compounds, which do not interfere with the basic operation of this invention in isolating the principal alcohol components of the mixtures.

Typical crude ethanol and propanol cuts obtained from the water layer of a Fischer synthesis process contains the following:

TABLE II Ethanol cut f OllS Compound B Pt" e.5 methyl alc 64. 7 n-butyraldehyde 75. 7 ethyl mm. 17. 1 ethyl alcohol 78. 5 methyl ethyl ketone 79. 6 isopropyl alcohol; 82. 3 t-buty almhnl 82.8 normal prop 97. 2 methyl propyl time 101. 7 nmtnl 103.2 water 100.0

TABLE m Normal propanol cut Binary pa e M Component Bomn Azeotrope Boiling Point, "0.

92.5 82 n-Valersldehyde. 103. 7 80. 6 Methyl n-Butyl Ketone 1 127.2

. Ternary with ethyl alcohol.

In such crude ethanol cuts, the kinds and relative quantities of the components vary greatly but the major components are generally ethyl alcohol, isopropyl alcohol and methyl ethyl ketone. Repeated fractional distillations of the ethanol cuts were found to be of no avail for eifecting isolation of pure ethyl alcohol or pure isopropyl alcohol.

In each of the above cuts the kinds and relative quantities of the components vary greatly but the major components for the ethanol cut are ethyl alcohol, isopropyl alcohol and methyl ethyl ketone and for the propanol cut, they are propanol, secondary butanol and methyl normal propyl ketone. When these cuts are distilled from aqueous solutions, many or the compounds form azeotropes with water and with themselves with the result that the boiling points are brought so close together that separation is very difllcult to achieve. The difflculties encountered can be appreciated by reference to Table II which shows the overlapping or the boiling points.

To obtain the desired separation of purified organic components from mixtures like those mentioned with benefits of the present invention, the mixture is subjected to a continuous fractional distillation in a column oi practical size, including a primary rectification zone, a secondary rectification zone above the primary zone and a stripp zone below the primary zone for countercurrent vapor-liquid contact under reboiling and refluxing conditions. A sumciently large quantity of sulfolane is introduced at the upper part of the primary rectification zone to effectively modify the relative volatilities of the organic compounds to be separated and to distill a larger part of one component or group of components than of another component or another group of components from the internal reflux.

The separation can be effected in a continuous manner under steady state conditions to obtain product streams of desired purities and constant compositions while supplying the large quantity of sulfolane to the upper part or the rectification zone. The temperature of the sulfolane introduced into the primary rectification zoneis preterably close to the temperature of the liquid on its feed plate, although it may be lowered to partially condense vapors ascending to the solvent feed plate.

Since the eflicient operation is essentially continuous, the sulfolane is added continuously near the'top of the primary rectification zone of the column while the mixture of oxygenated organic compounds to be separated is fed continuously into the column at a lower point while suflicient heat is provided to afiord distillation throughout the column;

"The feed stream of the oxygenated organic compounds is preferably introduced into the fractionating column between the primary rectias a feed stream at the bottom part of the primary rectification zone in the iractionating column pass up through the primary rectification zone in contact with descending internal liquid reflux under practically equilibrium reboiling and refluxing conditions. The secondary rectification zone serves to strip sulfolane from the overhead vapors.

The quantity of sulfolane required to be introduced continuously at the upper part of the primary rectification zone for accomplishing the desired separation of the close-boiling compounds is considerably greater than the quantity of condensate with which it becomes homogeneously mixed. This is necessary in order to make the sulfolane concentration of the internal reflux substantailly above a critical minimum in the range of 70-99 volume percent. With adequate sulfolane concentration in the internal reflux for eifecting the separation, the organic component to be isolated in the bottomsis dissolved in the internal reflux that reaches the bottom part of the primary rectification zone and finally the bottom of the stripping zone.

The minimum concentration in the internal reflux of the sulfolane for obtaining the separation depends on the particular organic compounds to be separated and varies between '70 and 99 volume percent. In a limiting case of isolating n-propyl alcohol from sec-butyl alcohol, essentially no separation is effected if the internal reflux contains less than 20 volume percent sulfolane, and for obtaining satisfactoryv results on a practical scale, more than 85 volume percent sulfolane, preferably 85-99 volume percent, is required in the internal liquid reflux. As

the dilution of the internal reflux becomes infinite, the selectivity of separation is increased but the operating efiiciency is excessively lowered on account of the relatively small quantities of the oxygenated organic compounds being processed.

Under steady state conditions existing in a continuously operating fractional distillation zone, the internal reflux having adequate concentration for accomplishing the separation of the close-boiling alcohols and other oxygenated compounds, there tends to be a nearly constant sulfolane concentration in the homogeneous liquid phase on each plate above the feed point and on each plate below the feed point although the average concentration on the plates above and below the feed point maydifler. This internal reflux in flowing from the top to the bottom becomes richer in the oxygen compounds having the lowest relative volatility in the presence of the sulfolane while the oxygen compounds having the highest relative volatility in the sulfolane are distilled overhead.

The overhead vapors from the secondary rectification zone are enriched in one or more of the organic components rendered relatively more volatile b the sulfolane concentration in the liquid reflux while the remaining portion of the organic material introduced with the feed remains dissolved in the internal reflux.

For example, in distilling an aqueous mixture of two alcohols the distillation may be carried out so that either one of the alcohols is obtained free of the other. In separating ethanol from isopropanol, the distillation may be conducted so that ethanol is obtained overhead and a mixture of ethanol and isopropanol is obtained in the bottoms or a portion of the isopropanol may be 6 taken overhead with the ethanol whereby isopropanol free of ethanol is obtained in the bottoms. A mixture of normal propanol and secondary butanol or other binary mixtures may be separated in the same manner.

The functioning of the stripping zone may be described as follows:

The mixture of the close-boiling alcohols and other oxygenated compounds to be separated, as in the liquid reflux from the bottom of the rectification zone, flows downwardly through the stripping zone in countercurrent contact with ascending vapors evolved from the solution under reboiling conditions. A sufllciently high concentration of sulfolane is maintained in the liquid flowing down through the stripping zone, as in the rectification zone, to make the liquid progressively richer in oxygenated compounds having the lowest relative volatility in the sulfolane while the oxygenated compounds having the highest relative volatility in the sulfolane are stripped from the liquid. Under practically equilibrium reboiling and refluxing conditions for complete strippingvin the stripping zone, the organic compounds rendered more volatile may be removed as vapor overhead from the stripping zone at the same ratethat they enter the stripping zone as part of the liquid feed to this zone and a solution of the organic compounds rendered less volatile freed of the more volatile compounds in the liquid may be withdrawn from a bottom part of the stripping zone.

In the accomplishment of the foregoing and related ends, the invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail certain illustrative embodiments of the invention, these being indicative. however, of but a few of the various ways in which the principle of the invention may be employed.

This invention will be described in detail as applied to the separation of n-propyl alcohol and sec-butyl alcohol from aqueous solution.

Referring to the drawing, a feed fraction is introduced by line 3 into the tower l where it is fractionated in the presence of a liquid stream of a sulfolane introduced through line 4, at a point several plates below the top of the tower. The conditions in the tower are such as to cause a distillation of the alcohol compounds in the presences of sulfolane on each plate of the tower. A sufilcient amount of sulfolane is added so that it is present to the extent of volume percent on each plate. As the vapors of the feed pass up the column some of them are dissolved in the large excess of sulfolane descending the column and are collected together with the sulfolane in pools on each plate. Conditions are maintained on each plate of the tower such that the liquid mixtures of the n-propyl and sec-butyl alcohols. are at their boiling points and are continuously being contacted with vapors boiled from the plates below. Because of the enhanced volatility of the n-propyl alcohol in relation to the secbutyl alcohol the vapors are relatively richer in. the former and poorer in the latter. By main taining the amount of sulfolane on each plate so large that infinite dilution is approached, the optimum relative volatilities for the separation of the desired components can be secured. Furthermore, by controlling the amount of oxy genated compound reflux and consequently the.

reflux ratio and the number of plates, the actual.

degree of separation may be varied until the desired product purity and recovery are obtained. Thus, suitable temperature and reflux conditions are maintained in the tower so that substantially pure n-propyl alcohol appears in the overhead stream and a solution of sec-butyl alcohol in sulfolane appears in the bottoms product. The plates above the point of sulfolane entry serve to; strip sulfolane from the alcohol overhead. Any water present in the feed will appear with the overhead alcohol product.

Overhead vapors consisting substantially of :pure n-propanol and all the water entering with the feed are withdrawn from the top of column I through line by which they are passed through condenser 6 to a receiver I. A portion of the condensation collected in receiver 1 is returned to the top part of the column I as external reflux through line 8. The remaining portion of distillate collected in receiver 1 is withdrawn through line 9 as a product.

Bottoms liquid consisting of a solution of secbutyl alcohol in sulfolane collected at the lower part of column I is passed by line In into reboiler I l for heating by indirect or direcu heat exchange with a heating medium such as live steam. A portion of the bottoms liquid heated and partially vaporized in the reboiler II is recycled by line I! to the lower part of column I. The remaining portion is withdrawn through line I3 to tower 2 where'sec-butyl alcohol is separated from the sulfolane and removed overhead through line H. Sufolane is removed from the bottom of the tower through line I5 and recycled to tower l.

Representative data are given in the following table showing the application of thi invention to the separation of mixtures of normal propyl alcohol and secondary butyl alcohol. The relative volatility given inthe table is the volatility of one component divided by that of the other, the volatility of each component being proportional to its partial pressure divided by its mol fraction in the liquid phase. It is also defined by the equation alpha;(y1/y2) /($l/IE2) where 1/ refers to the vapor phase mol fractions of the components to be separated and .2: refers to the liquid phase mol fractions of the components to be separated, SUbSCIlIJJ one designates the more volatile component and subscript two the less volatile component.

Run N0 543-106 543-110 Charge:

Mol Per Cent n-PrOH (Binary Basis) M01 Per Cent sec-BuOH (Binary Basis) Vol. Per Cent Sulfolanm,

Mol Per Cent sec-BuOH Liquid Sample:

Mol Per Cent n-PrOH Mol Per Cent sec-BuOH Relative Volatility of n-PrOH over scc-Bu0H Normal Volatility in absence of solvent: n-PrOH over sec-BuOH l.

TABLE IV Volatility of n-propanol relative to s-butanol in the presence of 90 vol. per cent dimethyl sulfolane Run No 560-4 669-6 Charge:

Mol per cent n-PrOH Binary Basis) 77. 6 67.9 Mol er cent sec-BuO (Binary Basis) 22. 4 32. 1 Vol. er Cent Dimethyl sulfolane 90 90 Vapor Sample:

Mol per cent n-PrOH 80. 1 69.0 Mo] per cent sec-BuOH l9. 9 31. 0 Liquid Sample:

Mol per cent n-PrOH 75. 5 67. 0 Mol per cent sec-BuOH 24. 6 33.0 Relative V olatility oi n-PrOH over sec-Bu0H.--- 1. 31 1.10 Normal Volatility in absence of solvent: n-PrOH over sec-BuOH l. 05

The nature and objects of the present invention having been thus fully set forth and specific examples of the same given, what is claimed as new and useful and desired to be secured b Letters Patent is:

1. The method of separating a mixture of ethyl alcohol and isopropyl alcohol which comprises continuously introducing a feed mixture of ethyl and isopropyl alcohols to a fractional distillation zone wherein vapors of said alcohols ascend countercurrently to a liquid reflux of the alcohols dissolved in a high proportion of sulfolane to increase the volatilny of the ethanol relative to the isopropanol, continuously removing vapors of ethanol and isopropanol overhead and a solution of isopropanol in sulfolane as bottoms.

2. The method of separating a mixture of ethyl alcohol and isopropyl alcohol which comprises continuously introducing a feed mixture of ethyl and isopropyl alcohols to a fractional distilladon zone wherein vapors of said alcohols ascend countercurrently to a liquid reflux of the alcohols dissolved in a high proportion of sulfolane to increase the volatility of the ethanol relative to the isopropanol, continuously removing vapors of ethanol overhead and a solution of ethanol and isopropanol in sulfolane as bottoms.

3. The method of separating a mixture of normal propyl alcohol and secondary butyl alcohol which comprises continuously introducing a. feed mixture of normal propyl and secondary butyl alcohols to a fractional distillation zone wherein vapors of said alcohols ascend countercurrently to a liquid reflux of the alcohols dissolved in a high proportion of sulfolane to increase the volatility of the normal propyl alcohol relative to the secondary butyl alcohol, continuously removing vapors of normal propyl alcohol and secondary butyl alcohol overhead and a solution of secondary butyl alcohol in sulfolane as hottoms.

4. The method of separating a mixture of normal propyl alcohol and secondary butyl alcohol which comprises continuously introducing a feed mixture of normal propyl and secondary butyl alcohols to a fractional distillation zone wherein vapors of said alcohols ascend countercurrently to 4' a liquid reflux of the alcohols dissolved in a high proportion of sulfolane to increase the volatility of the normal propyl alcohol relative to the secondary butyl alcohol, continuously removing vapors of normal propyl alcohol overhead and a solution of normal propyl alcohol and secondary butyl alcohol in sulfolane as bottoms.

5. In a process of separating aqueous azeotropic mixtures comprising normal propyl and secondary butyl alcohol, the steps which comprise continuously passing a solution of normal propyl 9 alcohol in sulfolane containing secondary butyl alcohol down through a stripping zone so that a liquid portion of the solution flows countercurrently in contact with vapors evolved therefrom under constant refluxing and reboiling conditions, maintaining a sufficiently high concentration of solvent in the resulting internal reflux to effect vaporization ofa larger part of the normal propyl alcohol than of the secondary butyl alcohol in said reflux, continuously withdrawing vapors of alcohols overhead from the stripping zone, the normal propyl alcohol being thus withdrawn as vapor at essentially the same rate that the normal propyl alcohol dissolved in said solvent enters the stripping zone and withdrawing from a bottom part of the stripping zone a solution of secondary butyl alcohol in the sulfolane substantially free of normal propyl alcohol and water.

6. The method of separating a mixture comprising normal propyl alcohol and secondary butyl alcohol which comprises continuously introducing a feed mixture of said alcohols to a fractional distillation zone wherein vapors of said alcohols ascend countercurrently to a liquid reflux of the oxygenated compounds dissolved in 90 volume percent of sulfolane to effect vaporization of the normal propyl alcohol, continuously removing vapor of the normal propyl alcohol overhead, and continuously removing a solution of the secondary butyl alcohol in the sulfolane as bottoms.

7. The method of separating a mixture of normal propyl alcohol, secondary butyl alcohol and isobutyl alcohol which comprises continuously introducing said mixture into a fractional distillation zone wherein vapors of said alcohols ascend countercurrently to a liquid reflux of the alcohols dissolved in 85-99 vol. percent of sulfolane to increase the volatility of the normal propyl alcohol relative to the secondary butyl alcohol and iso- 10 secondary butyl alcohol and isobutyll alcohol in sulfolane as bottoms from the fractional distillation zone.

8. The method of separating a mixture of normal propyl and butyl alcohols which comprises continuously introducing said mixture to a fractional distillation zone wherein vapors of said alcohols ascend countercurrently to a liquid reflux of the alcohols dissolved in -99 vol. per cent of sulfolane to increase the volatility of the normal propyl alcohol relative to the butyl alcohols, continuously removing vapors of normal propyl alcohol overhead and a solution of the butyl alcohols in sulfolane as bottoms from the fractional distillation zone.

9. The method of separating a first saturated aliphatic alcohol containing 2 to 4 carbon atoms per molecule from a higher molecular weight saturated alcohol containing 3 to 5 carbon atoms per molecule in a mixture containing the same which comprises introducing the mixture into a fractional distillation zone wherein vapors of said alcohols ascend countercurrently to a liquid.

reflux of the alcohols dissolved in a high proportion of sulfolane to increase the volatility of the first alcohol relative to the higher molecular weight alcohol, continuously removing vapors of the first alcohol overhead and a solution of the higher molecular weight alcohol in sulfolane as bottoms from the fractional distillation zone.

CARL S. CARLSON. PAUL V. SMITH, JR.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 2,360,859 Evans et a1 Oct. 24, 1944 2,379,110 Souders June 26, 1945 

1. THE METHOD OF SEPARATING A MIXTURE OF ETHYL ALCOHOL AND ISOPROPYL ALCOHOL WHICH COMPRISES CONTINUOUSLY INTRODUCING A FEED MIXTURE OF ETHYL AND ISOPROPYL ALCOHOLS TO A FRACTIONAL DISTILLATION ZONE WHEREIN VAPORS OF SAID ALCOHOLS ASCEND COUNTERCURRENTLY TO A LIQUID REFLUX OF THE ALCOHOLS DIS- 